The present invention relates to controlling the phase and beam pattern of individual elements in antenna arrays, and, in particular, relates to controlling the phase and beam pattern of the individual elements by means of diffracted light energy.
Radar and radio beams need to be directed, both to find targets and to transfer information effectively. In military environments, directing and shaping the electromagnetic beam help shield friendly signals from detection and reduce the impact of hostile jamming. In wireless communications, transmission quality can be affected by beam pattern. Beam pattern control therefore allows radar and radio equipment to operate more efficiently, thereby saving weight and power.
An antenna in increasing use is the microstrip, which consists of metal foil patterns on a dielectric substrate. Microstrip antennas are efficient. They have a low profile, permit a wide variety of antenna types, and are relatively easy to manufacture. Conformal arrays (that is, arrays shaped to an object) of microstrip antenna elements transmit microwaves in many military systems. In one application, an omnidirectional microstrip antenna wraps a small cylindrical missile body section (Richard C. Johnson editor, Antenna Engineering Handbook, 3 ed. (New York, McGraw-Hill Inc., 1993), 7-1-7-30). Multiple-element antennas, phased-array microstrip antennas that incorporate input phase shifters, have also been developed to shape beam patterns and provide electronic beam scanning.
These antenna arrays operate on the basis of wave interference among output signals from each element (Reference Data for Radio Engineers, 5 ed. (Indianapolis Ind., Howard W. Sams Co., October 1968), 20-25). By controlling the characteristics of the electromagnetic wave, such as phase and amplitude, emitted by individual elements, the overall beam pattern and orientation of the antenna can be modified to meet specific needs. Adjusting the shapes and location of beam lobes, for example, can effectively "null out" a jammer trying to disrupt radar target detection or radio communications. Controlling the individual elements electronically also allows the main beam of the antenna to scan a wide area without physically rotating. Electronic control of the antenna structure provides faster operation and greater reliability than mechanical scanning or rotation. However, controlling individual elements electronically requires each antenna element to have an electronic phase shifter. These phase shifters substantially increase the weight of and power required by the system, and thus they reduce its reliability.
Optical time-delay networks can replace phase shifters. Optical taps convert signal phase differences to time delays, thereby moving the antenna beam pattern to null out multipath jamming interference (M. E. Turbyfill and J. M. Lutsko, Anti-Jamming Optical Beam Nuller, In-House Report RL-TR-96-65 (Rome Laboratory, May 1996)). Optical control promises higher operating speed, and it reduces the tendency of the beam to wander as the frequency changes (so-called radar beam `squint`). However, optical control requires both considerable computation and a complex electro-optical structure. Such a structure is costly to produce and operate, and it is sensitive to vibration.
Apparatus for controlling the phase and polarization of individual antenna elements was disclosed in U.S. Pat. No. 4,053,895 to Malagisi (1977), the disclosure of which is incorporated herein by reference. Malagisi teaches providing switchable shorting circuits between a common ground plane and the disc antenna elements. In an early embodiment of Malagisi's teaching, metal bolts were raised or lowered to change the circuit. In a later embodiment, the forward or reverse bias of pairs of diodes was controlled to implement open- and short-circuit combinations for each antenna element in the array. This concept was extended in U.S. Pat. No. 4,367,474 to Schaubert et al. (1983) to include computer control of the switching diodes. U.S. Pat. No. 4,751,513 to Daryoush et al. (1988) added discrete photo-diodes that perform the switching action with energy from light. All of the prior art structures rely on fixed componentry and are therefore limited in their ability to provide the flexibility required for modem wireless communication and microwave sensor systems.
Thus there exists a need for a continuously reconfigurable apparatus to control the phase, polarization, and frequency of individual antenna elements that is simple, inexpensive, easy to implement, and substantially insensitive to vibration.